Escherichia coli is the predominant facultative anaerobe in the gastrointestinal tracts of mammals. Yet the essence of how E. coli colonizes its mammalian hosts is not understood. According to the nutrient-niche hypothesis, the numerous ecological niches in the intestine are defined by nutrient availability and species coexist by competing for one or a few limiting nutrients. This hypothesis suggests that commensal E. coli strains might act as a first line of defense against enteric E. coli pathogens, but it has not been tested in that light. In this competing renewal application, we propose to extend our investigation of the nutritional basis for intestinal colonization by testing the hypothesis that mucus-derived nutrients provide nutritional niches occupied by competing E. coli commensal and pathogenic strains. The first specific aim will test the hypothesis that different E. coli commensal and pathogenic strains preferentially use different nutrients for colonization of the mouse intestine. Work described in the progress report suggests that different E. coli strains use different nutrients for colonization. For strains representing the four phylogenetic groups of E. coli, the specific nutrient requirements during the initiation and maintenance stages of colonization will be determined by using strategies developed for the currently funded project, i.e., genes induced on DNA arrays under conditions designed to mimic the mouse intestine will be knocked out and the mutants tested in competition with their wildtype parent to determine their relative fitness for colonization. The second specific aim will test the hypothesis that preferential utilization of different nutrients and/or the ability to switch to alternative nutrition is the basis for co-colonization of E. coli strains representing each of the four phylogenetic groups. The corollary hypothesis will also be tested, that direct competition for the same nutrient leads to decreased fitness for colonization of one of the strains. These studies may help explain differences in human susceptibility to infection by E. coli pathogens. [unreadable] [unreadable]